one of the best way specially if you don't like fishing is in buying turnips and selling them. you could have the mansion in three weeks in that way, you could in the same time make growing trees of another fruit that yours and sell everything, with a lot of trees you could have a lot of money too.
Hands down, the best way takes the longest to start, but after it's started, it's just pretty much collecting every three days to get crazy profit.
I think you all know what I'm talking about: foreign fuit.
To the first poster way above, if you didn't already know (you may, though), getting fruit not native to your town sells for 500 bells instead of 100 bells. 3 to a tree. 1500 bells instead of 300 bells. Now, plant lots of foreign fruit. Dozens. 10 foreign fruit trees will yield 15,000 bells after they all grow.
And since they all grow after 3 days, you can get more than 80,000+ every week. Way more.
I know that was all rudimentary to veteran players, but hopefully a newbie to AC will read it. It's what made me money on GCN, DS, and now hopefully Wii. I already have 3+ pear trees growing. All is going according to plan *rubs hands*